1. Field of the Invention
This invention relates to printed circuit substrates incorporating impedance elements, and more particularly to a printed circuit substrate which has a pattern of impedance elements and a conductor pattern incorporated on an insulating support. In this manner a printed circuit board with impedance elements can be manufactured to a high degree of tolerance.
2. Description of the Related Art
In one conventional method of producing a printed circuit board, an insulating support is provided with a resistance layer on the entire surface of the support, and a highly conductive material layer is positioned on the resistance layer. Using the conventional method, printed circuit substrate, an insulating region, a resistance region, and a conductor region are formed by a subtractive mask-etching method. In another prior art method, a printed circuit substrate is manufactured by covering a highly conductive material layer such as a copper foil with a removable masking sheet, and a resistance layer is formed on the other surface of the highly conductive material layer by electrodeposition. Then, the masking sheet is removed, and thereafter an insulating support is combined with the resistance layer. Thereafter, the surface of the copper foil is covered with photoresist, imagewise exposed to a conductor pattern and a resistor pattern in combination and developed such that the photoresist remains in the pattern regions. The copper foil in the region which is not covered with the photoresist is removed by etching, and the resistance layer exposed is removed by using an etching solution, as a result of which the surface of the insulating support is exposed. Then, the left photoresist is removed by using a removing solution. Thereafter, the substrate is covered with photoresist again, exposed through a photographic negative having the conductor pattern and the substrate developed to retain the photoresist in the conductor pattern region. The copper foil in the region which is not covered by the photoresist is removed by etching, as a result of which the surface of the resistance layer corresponding to the resistor pattern region is exposed. Then, the remaining photoresist is removed by using a removing solution. A solder stop-off or the like is applied to the resistance layer in the resistor pattern region by printing, and is then heated and cured to cover the resistance layer to achieve a printed circuit board with resistors. A drawback in the prior process is that, the resistance layer is thin and its mechanical strength is very low. The multiple developing steps thus causes variations in the sheet resistance and a variety of other characteristics.
In order to overcome these various difficulties, another method has been proposed in which the conductor pattern region of the circuit substrate is protected by a gold plating film, and in the final process the copper foil having a configuration corresponding to the resistor pattern region is removed by etching. However, persons skilled in the art will readily understand that in this method the processes are rather intricate requiring considerable skill. Another method of forming a printed circuit substrate is described in U.S. Pat. No. 4,368,252. This technique forms a resistor pattern film and a conductor pattern film on the two surfaces of a copper foil in a predetermined position. An insulating support is combined directly or indirectly with the resistor pattern film on the high conductive material layer. As is apparent from the above that conventional printed circuit substrate processing technique suffer from various disadvantages including a large number of intricate processing steps. Thus it takes a relatively long period of time for the processing, the manufacturing cost is high, the yield is limited, and the auxiliary material cost is high. In addition, embedded resistors made by using a resistive foil have electrical tolerances which are too high, i.e. on the order of xc2x110% of their desired values. A significant part of this variation is due to the inability to resolve the resistive pattern within tighter tolerances. This invention solves these problems.
According to the present invention, a printed circuit substrate with pre-formed impedance elements on a sheet of an electrically highly conductive material, attached to a support is manufactured. An insulating sheet is applied to the sheet of conductive material with the impedance elements therebetween. After removing the support from the sheet of conductive material the sheet of conductive material is etched to form a desired pattern such that at least some of the conductive lines contact at least some of the impedance elements. Optionally the another pattern of impedance elements and conductive lines are formed on an opposite side of the insulating sheet. As a result, impedance elements are formed on the insulating sheet which have much improved electrical tolerances.
The invention provides a process for forming a printed circuit substrate with impedance elements comprising performing steps (a) and (b) in either order:
(a) depositing a layer of an impedance material on a first surface of a sheet of an electrically highly conductive material;
(b) attaching a second surface of the sheet of highly conductive material to a support; then
(c) applying a layer of a photoresist material onto the layer of impedance material;
(d) imagewise exposing the photoresist material thus forming image and nonimage areas, and then removing the nonimage areas while retaining the image areas;
(e) etching away the portion of the impedance layer material underlying the removed nonimage areas of the photoresist material, and then optionally removing the image areas of the photoresist material thus leaving a pattern of impedance elements on the sheet of highly conductive material.
Preferably the process further comprising the subsequent steps of:
(f) forming a plurality of target holes through the sheet of highly conductive material;
(g) attaching one side of a sheet of an insulating material to the sheet of highly conductive material such that the impedance elements are between the sheet of insulating material and the sheet of highly conductive material;
(h) removing the support from the sheet of highly conductive material;
(i) applying an additional layer of a photoresist material onto the second surface of the sheet of highly conductive material;
(j) imagewise exposing the additional layer of photoresist material thus forming image and nonimage areas, and then removing the nonimage areas while retaining the image areas;
(k) etching away the portion of the sheet of highly conductive material underlying the removed nonimage areas of the photoresist material, and then optionally removing the image areas of the photoresist material thus leaving a pattern of conductive lines of the highly conductive material such that at least some of the conductive lines contact at least some of the impedance elements.
More preferably the process provides the additional steps of performing steps (l) and (m) in either order:
(l) depositing a second layer of an impedance material on a first surface of a second sheet of electrically highly conductive material;
(m) attaching a second surface of the second sheet of highly conductive material to a support; then
(n) applying a layer of a photoresist material onto the second layer of impedance material;
(o) imagewise exposing the photoresist material thus forming image and nonimage areas, and then removing the nonimage areas while retaining the image areas;
(p) etching away the portion of the second layer of impedance material underlying the removed nonimage areas of the photoresist material, and then optionally removing the image areas of the photoresist material thus leaving a pattern of second impedance elements on the second sheet of highly conductive material;
(q) forming a plurality of target holes through the second sheet of highly conductive material;
(r) attaching the second sheet of highly conductive material to another side of the sheet of an insulating material such that the second impedance elements are between the layer of insulating material and the second sheet of highly conductive material;
(s) removing the second support from the second sheet of highly conductive material;
(t) applying an additional layer of a photoresist material onto the rear side of the second sheet of highly conductive material;
(u) imagewise exposing the additional layer of photoresist material thus forming image and nonimage areas, and then removing the nonimage areas while retaining the image areas;
(v) etching away the portion of the second sheet of highly conductive material underlying the removed nonimage areas of the photoresist material, and then optionally removing the image areas of the photoresist material thus leaving a pattern of second conductive lines of the highly conductive material such that at least some of the second conductive lines contact at least some of the second impedance elements.
Another embodiment of the invention provides performing steps (a) and (b) in either order:
(a) depositing a pattern of impedance elements on a first surface of a sheet of electrically highly conductive material;
(b) attaching a second surface of the sheet of electrically highly conductive material to a support;
(c) forming a plurality of target holes through the sheet of highly conductive material;
(d) attaching one side of a sheet of an insulating material to the sheet of highly conductive material such that the impedance elements are between the sheet of insulating material and the sheet of highly conductive material;
(e) removing the support from the sheet of highly conductive material;
(f) applying a layer of a photoresist material onto the second surface of the sheet of highly conductive material;
(g) imagewise exposing the layer of photoresist material thus forming image and nonimage areas, and then removing the nonimage areas while retaining the image areas;
(h) etching away the portion of the sheet of highly conductive material underlying the removed nonimage areas of the photoresist material, and then optionally removing the image areas of the photoresist material thus leaving a pattern of conductive lines of the highly conductive material such that at least some of the conductive lines contact at least some of the impedance elements.
Preferably this embodiment further comprising performing steps (i) and (j) in either order:
(i) depositing a second pattern of impedance elements on a first surface of a second sheet of electrically highly conductive material;
(j) attaching a second surface of the second sheet of highly conductive material to a second support; then
(k) forming a plurality of target holes through the second sheet of highly conductive material;
(l) attaching the second sheet of highly conductive material to another side of the sheet of an insulating material such that the second impedance elements are between the layer of insulating material and the second sheet of highly conductive material;
(m) removing the second support from the second sheet of highly conductive material;
(n) applying an additional layer of a photoresist material onto the rear side of the second sheet of highly conductive material;
(o) imagewise exposing the additional layer of photoresist material thus forming image and nonimage areas, and then removing the nonimage areas while retaining the image areas;
(p) etching away the portion of the second sheet of highly conductive material underlying the removed nonimage areas of the photoresist material, and then optionally removing the image areas of the photoresist material thus leaving a pattern of second conductive lines of the highly conductive material such that at least some of the second conductive lines contact at least some of the second impedance elements.
A third embodiment of the invention provides a process for forming a printed circuit substrate with impedance elements comprising performing steps (a) and (b) in either order:
(a) depositing a layer of a photosensitive impedance material on a first surface of a sheet of an electrically highly conductive material;
(b) attaching a second surface of the sheet of highly conductive material to a support; then
(c) imagewise exposing the photosensitive impedance material thus forming image and nonimage areas, and then removing the nonimage areas while retaining the image areas thus leaving a pattern of impedance elements on the sheet of highly conductive material;
(d) forming a plurality of target holes through the sheet of highly conductive material;
(e) attaching one side of a sheet of an insulating material to the sheet of highly conductive material such that the impedance elements are between the sheet of insulating material and the sheet of highly conductive material;
(f) removing the support from the sheet of highly conductive material;
(g) applying a layer of a photoresist material onto the second surface of the sheet of highly conductive material;
(h) imagewise exposing the layer of photoresist material thus forming image and nonimage areas, and then removing the nonimage areas while retaining the image areas;
(i) etching away the portion of the sheet of highly conductive material underlying the removed nonimage areas of the photoresist material, and then optionally removing the image areas of the photoresist material thus leaving a pattern of conductive lines of the highly conductive material such that at least some of the conductive lines contact at least some of the impedance elements.
This embodiment preferably further provides performing steps (j) and (k) in either order:
(j) depositing a second layer of a photosensitive impedance material on a first surface of a second sheet of electrically highly conductive material;
(k) attaching a second surface of the second sheet of highly conductive material to a second support; then
(l) imagewise exposing the photosensitive impedance material thus forming image and nonimage areas, and then removing the nonimage areas while retaining the image areas thus leaving a pattern of second impedance elements on the second sheet of highly conductive material;
(m) forming a plurality of target holes through the second sheet of highly conductive material;
(n) attaching the second sheet of highly conductive material to another side of the sheet of an insulating material such that the second impedance elements are between the layer of insulating material and the second sheet of highly conductive material;
(o) removing the second support from the second sheet of highly conductive material;
(p) applying a layer of a photoresist material onto the rear side of the second sheet of highly conductive material;
(q) imagewise exposing the layer of photoresist material thus forming image and nonimage areas, and then removing the nonimage areas while retaining the image areas;
(r) etching away the portion of the second sheet of highly conductive material underlying the removed nonimage areas of the photoresist material, and then optionally removing the image areas of the photoresist material thus leaving a pattern of second conductive lines of the highly conductive material such that at least some of the second conductive lines contact at least some of the second impedance elements.